Time described those who know him best as saying Mueller “is the kind of man who flicks the lights off and on at his home to inform guests that it’s time to leave a social gathering.”

Mueller, a seasoned federal prosecutor who was the FBI director for 12 years, was appointed in May 2017 by Deputy Attorney General Rod Rosenstein to lead the investigation into Russian interference in the 2016 US election and whether President Donald Trump’s campaign collaborated with Moscow to tilt the race in Trump’s favour.

Since taking over the investigation, Mueller’s team has brought charges against five Americans once affiliated with Trump’s campaign or administration, as well as 13 Russian nationals, 12 Russian intelligence officers, three Russian companies, and two other people.

As Mueller’s investigation closes in on some of Trump’s top allies, friends and former colleagues who spoke to Time described Mueller as fiercely loyal to procedure and duty-driven – qualities they said stemmed from his serving in the Marine Corps in the Vietnam War after college.

Mueller is also notoriously tough on his subordinates. According to the Time profile, he was known to chew out FBI special agents whose work didn’t match his standards, and he has acknowledged he would interrupt attorneys at the Justice Department by asking “What is the issue?” when they didn’t explain things concisely enough.

But Mueller didn’t fare as well when he interjected with that same question while his wife of 52 years, Ann, was telling him about a tough day she had, as he recounted in a 2013 commencement address at the College of William & Mary.

“I am your wife,” Mueller recalled her replying, according to Time. “I am not one of your attorneys. Do not ever ask me, ‘What is the issue?’ You will sit there and you will listen until I am finished.”

“That night, I did learn the importance of listening to those around you – truly listening – before making judgment, before taking action,” Mueller added. “I also learned to use that question sparingly, and never, ever with my wife.”